Where do they hoard this stash?
That, like everything else the industry does, they do not share. I knew a highly respected mining historian, and even he was impressed at how secretive they are. He was used to talking with miners, who are usually very gregarious; but once inside the mining company office the level of secrecy was stifling.
But as a hint of this, when the telecom companies upgraded their national network from copper wire to fiber optic, there was enough surplus copper to, as the historian put it, build several aircraft carriers of pure copper. But the market price didn’t vary one cent, because none of it went on to the market.
A lot of it went back to Anaconda Copper, one of the largest trusts of the Rockefeller family along with Standard Oil. Then, when WWII came around, copper was a key strategic metal, so the industry got into bed with the US government, and there it has remained.
Bottom line: while gold is kept under very close control, and silver is free enough so players like the Hunt brothers could manipulate it, copper is a no-go, and those who control it are utterly ruthless.